1. Field of the Invention
This invention relates to a sintered silicon nitride-silicon carbide composite material and a process for producing the same and, more particularly, to a sintered composite material excellent in toughness and strength, which is substantially formed of silicon nitride predominantly in .beta.-phase and about 5 to 35% by weight of silicon carbide predominantly in .beta.-phase and which has a microstructure in which the silicon carbide grains of 1 .mu.m or less in average size are uniformly dispersed. This invention further relates to a process for producing an amorphous composite powder suitable for the fabrication of said sintered material.
2. Description of the Prior Art
Silicon nitride has recently attracted increasing attention as an engineering ceramic for use in structural materials which are exposed to high temperature. Especially, silicon nitride is excellent in both heat shock resistance and fracture toughness, while silicon carbide is distinguished in both oxidation resistance and strength at high temperature. For these reasons, both silicon nitride and silicon carbide are now under development in fields where the advantages of each material might be fully manifested. For instance, in Japanese patent application "Kokai" (Laid-open) No. 201,663/86, it is attempted to improve the strength and toughness of a sintered silicon nitride by producing in the sintered product a fibrous material of large aspect ratio (length-to-breadth ratio) by controlling the sintering process. On the other hand, various attempts have been made to develop a sintered silicon nitride-silicon carbide composite material to take advantage of characteristics of both material.
As the methods for producing silicon nitride-silicon carbide composite materials, there may be mentioned the following ones.
(1) A method of pressure sintering, wherein a mechanical mixture of silicon nitride (Si.sub.3 N.sub.4) powder and silicon carbide (SiC) powder is sintered, for example, by means of a hot-pressing.
(2) A method of reaction sintering, wherein a molded body comprising silicon carbide (SiC) powder and silicon (Si) powder is subjected to nitriding reaction to form therein a silicon nitride-silicon carbide composite material or a molded product comprising silicon nitride (Si.sub.3 N.sub.4) powder and carbon is subjected to silicon (Si) permeation to form therein a silicon nitride-silicon carbide material.
(3) A method, wherein, a mixture of organo-silicon polymer and silicon (Si) powder is molded directly or after heat treatment and the molded body is subjected to a nitriding reaction to form a silicon nitride-silicon carbide composite material.
Of these methods, the methods (2) and (3) produce the materials generally having advantages of good dimensional precision and excellent moldability but difficulties are encountered in producing a high density products which are excellent in toughness and strength because of the tendency of the resulting sintered products to be porous.
For the above reasons, in order to obtain high density products, it is general practice to adopt method (1). For instance, in U.S. Pat. No. 4,184,882 or J. Am. Ceram. Soc., 56, 445 (1973), it is disclosed that Si.sub.3 N.sub.4 -SiC composite materials, improved in thermal conductivity and high-temperature strength, as compared with that obtained from silicon nitride (Si.sub.3 N.sub.4), can be obtained by adding silicon carbide (SiC) powder of less than about 5 .mu.m in particle size to silicon nitride (Si.sub.3 N.sub.4) powder. It is also shown, however, that the product has no tendency to improve in room-temperature strength as compared with the case of silicon nitride (Si.sub.3 N.sub.4), the tendency depending to a large extent on the particle size of the silicon carbide (SiC) which was added.
It is also disclosed in U.S. Pat. No. 3,890,250 that a silicon nitride-silicon carbide composite material high in both strength even at room temperature and electric conductivity can be obtained by using a silicon carbide powder of 3 to 5 .mu.m in particle size. In this U.S. Patent, however, no mention is made of the fracture toughness whichis one of the important physical properties. As will be shown later in a comparative example, a sintered silicon nitride-silicon carbide composite material having a fracture toughness and a room-temperature strength as high as those of the sintered composite material of the present invention can never be obtained by sintering a mere mechanical mixture of a fine silicon nitride (Si.sub.3 N.sub.4) powder and a fine silicon carbide (SiC) powder.
Further, in Japanese Patent Application "Kokai" (Laid-open) No. 58-91070, there is disclosed a sintered composite material excellent in high-temperature strength and heat shock resistance, which is formed from a silicon nitride (Si.sub.3 N.sub.4)-silicon carbide (SiC) mixed powder obtained by vapor phase reaction. This mixed powder, however, contains a halogen such as chlorine and, accordingly, it is impossible to obtain from such a mixed powder a high-performance composite material.
As described above, in spite of some partial improvement in physical properties, the conventional composite materials are unable to produce those sintered composite materials higher in strength and toughness which are the object of the present invention.
Under the above circumstances, the present inventors previously found an amorphous composite powder composed of silicon, carbon, nitrogen and hydrogen in a composition represented by SiCxNyHz, wherein 0&lt;x&lt;4, 0&lt;y&lt;3, and 0&lt;z&lt;4, which contains substantially no halogen and oxygen [Japanese patent application "Kokai" (Laid-open) No. 221,311/85]. When synthesized under proper conditions, this amorphous powder is a fine powder of submicron order in particle size. When prepared under most favorable conditions, it is a spherical fine powder of 0.2 to 0.05 .mu.m in particle size and has a narrow particle size distribution. Under the temperature conditions of the synthesis, the powder is thermally stable and change in composition is also very small. It was found, however, that this powder as such had a disadvantage of requiring a high temperature of 1,600.degree. C. or above for sintering and, accordingly, it tends to liberate gases during sintering, thereby leaving behind a porous product in place of a high density product, having a high strength, and a high toughness. In addition, it was found that it had another disadvantage of tending to react chemically with water (H.sub.2 O) or oxygen (O.sub.2).